The Houston area's unique climate is both an asset and a challenge to the herb gardener. Each month of the year there are herbs thriving and ready for harvest. Our basils and ornamental sages hold nationwide bragging rights. We can grow effortlessly the ginger and patchouli of which northern gardeners can only dream. We're flavoring our meals with fresh dill, parsley, and cilantro from the garden while they're digging out of snowdrifts.
On the other hand, we don't talk much about artemisias; most of us don't attempt tarragon more than once; and we nurse our lavender and culinary sage through storms and floods. But there are two "secrets" to successful herb gardening here. Time after time you have read, "full sun and well-drained soil" or "must have adequate drainage." The major problem to conquer, since we certainly have plenty of sun, is drainage. And the answer is raised beds made of stone, brick, old railroad ties, landscape timbers, or even treated 2" x 12" lumber. Anything that will raise the soil in your herb bed above the level of surrounding ground will work. Add lots of compost and a little sand to our thick gumbo soils and you'll certainly see a change. Put that mixture in a raised bed where the roots can grow deep into moist soil without becoming waterlogged and you'll see a miraculous difference.
The second "secret" is mulch. Pine bark, wood chips, rice hulls, shredded leaves, a thin layer of grass clippings, old rain-spoiled hay, pine needles, or strips of newspaper - any of those will work. Some people mulch with just peat moss, which eventually does decompose and help with soil building. But peat moss that is allowed to dry sheds rain like a shingle, which doesn't get the needed moisture to the roots of your herbs. Mixing different mulch materials, including peat moss if you wish, will solve the problem with packing and allow better aeration. Silver and gray plants, and plants with tiny leaves like sage, lavender, rosemary or thyme should be mulched with a non-organic mulch like coarse sand or crushed eggshells.
Keep a 2" - 4" layer of mulch on your beds at all times - it keeps roots cool and helps retain moisture in the summer heat as well as protecting the soil from erosion by wind or hard rain. Mulch keeps down competing weeds, keeps your herbs cleaner for harvest, and provides an insulating blanket for the roots during winter's cold.
The Herb Society of America - South Texas Unit